In a Pressurized Water Reactor (PWR), a certain number of linear motion devices are used for controlling the power of the reactor. Each device comprises a vertical control shaft connected to the upper end of the control rod and moved by electromagnetically actuated pawls. Such a device described, for example in the U.S. Pat. No. 3,158,766, allows a step by step movement of the control rod consisting of a cluster of neutron absorbing rods, in a fuel assembly of the core along the vertical direction. Each device is arranged in a sealed vessel in communication with the inside of the reactor vessel and mounted on its cover. A motion device, called control rod mechanism comprises two sets of pawls (or dogs), one set being mounted on a first support in the sealed vessel and the other being mounted on a second support movable in the vertical direction of the sealed vessel allowing the transfer of the control shaft and the control rod,
The control shaft is provided with grooves equally spaced in the vertical direction and each dog of each set comprises a tooth engageable with play in a groove by rocking of the pawl under the effect of an electromagnetic actuation or impulse. Each dog is articulated on its support through an upper horizontal axle and on a connecting rod through a lower horizontal axle. The connecting rod is articulated at its end opposite to the pawl on a part of the electromagnetic actuation means. The rocking of the pawl is obtained through the connecting element. The support of the transfer pawl is also actuated by an electromagnetic means for is displacement along a vertical distance equalling that separating two successive grooves of the control shaft and corresponding to one step of displacement.
The holding pawls are actuated to engage their teeth in a groove of the control shaft at the end of a displacement to allow the return of the transfer pawls to their original position. The transfer pawls are actuated to engage the control shaft before effecting a displacement of the control shaft.
After the engagements of one set of pawls, a slight upward displacement of their support allows a disengagement of the other set of pawls without any friction.
Generally speaking, every movement of the pawls discribed in U.S. Pat. No. 3,158,766 is made without friction by means of the play between the teeth of the pawls and the grooves of the control shaft. Thus, the wear on the motion device is greatly reduced.
However, when the nuclear reactor is used with frequent displacements of the control rods, the wear on the dogs may become excessive after a certain period of operation and what is worse, a tooth may break and be separated from the pawl.
The physical phenomena responsible for the wear and breakage of the pawls in the control rod mechanisms are not accurately known at this time, except the fact that shock and vibrations are important factors in such discontinuous mechanisms.
It is necessary for the mechanisms to be sufficiently reliable to bear at least 10 million stops without breakage and without an excessive wear, under the conditions of use involving large stresses such as in the high power reactors built in recent years (1300 MW), which is not possible with the mechanisms of the prior art.
It has been previously proposed to replace mechanisms with pawls by other types of mechanisms such as screw and nut or electromagnetic coupling mechanisms. However the design of the reactor would have to be partly modified and adapted to the new type of mechanism.